In data processing systems, it is common practice for different users to share a common resource such as a bus. Generally, the users are interconnected through a system bus. Each user has the capability of requesting the bus for receiving or/and sending information or requests from or to another user.
Thus, in this environment the accesses to the bus must be managed so that only one user is given access to the bus at one time.
There exist different arbitrating techniques which can be classified into two modes. In the first mode, called "fixed mode", each user is assigned a fixed priority and when the bus is free, the new bus preemption is gotten by the highest priority user requesting the bus. In the second mode, called "rotating mode", or "round-robin mode", when the bus is free, each user is granted access to the bus one by one until all pending requests are serviced. This second mode optimizes fairness between the users, but no protection against overrun conditions is provided.
European patent application 0039635 describes an arbitration system based on a hierarchical round-robin process wherein the working time is partitioned between the users which request the bus. A drawback of this system is that the users must be of the same nature and that all users are able to wait for their selection.